it would be very kind, if anybody could help me: Sigil (for mac) does not display the whole .htm document – only a fragment of it. This is, what I have done: I converted the word.doc into a htm.doc. Then I imported it into Sigil. When I open it there, only a fragment of the htm.doc is displayed. Did anyone has an idea, what may cause this?

Besides repeating the analysis others above have recommended, what I do in similar situations is to note the last visible line in Book View, then switch to Code view and look at the same place, noting any anomalies in the code in that vicinity. Sometimes it's as simple as a misplaced > or <, or a mis-spelled entity code.

Another approach would be to expand the epub, and run the .htm file through a browser to note whether the same problem exists. The Qt rendering engine used by Sigil is sometimes more finicky than a typical browser rendering engine, or so I've found.